Clostridium difficile is a leading cause of hospital-acquired illness in developed countries. Treating C. difficile infection (CDI) is complicated by antibiotic resistance, recurrence (RCDI), and hypervirulence of emerging strains that produce a highly toxic variant of TcdB. TcdB is a major C. difficile virulence factor that contributes to gastrointestinal damage, inflammation, and systemic damage. In these studies experiments will characterize the differences between the historical and milder form of TcdB (TcdB1) and the hypertoxic TcdB2. Published and preliminary data from our group shows that TcdB2 has a broader tropism and is more cytotoxic than TcdB1. In addition, antibodies to TcdB1 do not cross-neutralize TcdB2, suggesting that current vaccines and therapeutic antibodies targeting TcdB may not provide broad protection against multiple strains of C. difficile. The study will examine molecular differences between TcdB1 and TcdB2 to determine how these impact cellular intoxication, antigenicity, and influence RCDI. Aim 1 will characterize the differences in dual-receptor interactions between TcdB1 and TcdB2, assess how this impacts endocytosis, and identify key residues that influence these events. Aim 2 will explore a mechanism through which TcdB2 cloaks neutralizing epitopes which are otherwise exposed in TcdB1. Experiments in aim 3 of this project will determine how the differences in TcdB1 and TcdB2 predispose patients to RCDI. The results from this project will provide the first insights into the differentil effects of TcdB1 and TcdB2 and the impact of TcdB2's heightened toxicity and altered antigenicity on the emergence of hypervirulent strains of C. difficile.